In such a pressure casing, the casing shell sections have contact faces, which customarily are pressed against each other by means of a respective threaded connection outside the sealing zone, so that so-called parting joints are created between adjacent casing shell sections.
If three or more casing shell sections are to be interconnected at the same time, as exemplarily shown in FIG. 1 with four casing shell sections, then the intersection region of the parting joints, the so-called cross joints, cannot be reliably sealed without additional measures. At the contact point of the three or more adjoining casing shell sections, small disturbances in the required geometry, as, for example, they arise as a result of damage to the flange edges during assembly of the mostly heavy casing shell sections, are practically unavoidable, which, with the high pressures which occur inside a turbomachine, inevitably leads to leaks.
The prior art recognizes a plurality of solution proposals for sealing a cross flange with four casing shell sections which adjoin each other. The documents DE 27 55 356 or CH 625 025, as the case may be, disclose known solution proposals, according to which the parting joints of a pressure vessel, which extend in a cross-form manner, are sealed by means of a bolt in the intersection region of the parting joints. Welding of the abutting ends of the casing shell sections, as already known, is also described.
The use of a cover flange, which by means of a seal is sealed on a seating face which is incorporated in the casing shell sections, is described as new in the mentioned document.
All these known solution proposals, however, have disadvantages which counteract a reliable, maintenance-friendly, i.e. easily disassemblable and reassemblable, sealing of the cross flange:                1. The sealing of a cross flange by means of a bolt does not meet the requirement for a reliable, maintenance-friendly seal:                    The bolt has to be replaced during overhauls of the turbomachine, depending upon how far wear and corrosion have impaired its durability.            Since bolt and bolt seat as a rule are damaged when fitting and removing the bolt, a reworking, i.e. a reboring of the bolt seat, is necessary. At the same time, a new, larger bolt is required.            During thermal distortions of the casing, the seal by means of a bolt is prone to leaks, since the bolt has practically no elastic deformation in order to compensate distortions of the casing.                        2. The sealing of a cross flange by welding:                    Welding of the casing shell sections in the intersection region of the parting joints is indeed possible, however the taking apart of the casing becomes more difficult and overhauling of the turbomachine becomes more difficult as a result. Furthermore, reworking of the casing shell sections and their rewelding is necessary.                        3. Sealing by means of a flange in the intersection region of the parting joints:                    It is disadvantageous in this form of sealing that the seating face on the casing shell sections is susceptible to scratches and impact points, as they can easily arise during assembly of the mostly heavy casing shell sections. During thermal distortions or a displacement of the casing shell sections in a direction perpendicular to the flange plane, the sealing integrity is no longer ensured, so that the sealing face has to be reworked.                        